An "acoustic shadow" is when the sounds of an event—here, a battle—cannot be heard by people nearby—say, in the neighboring valley or a parallel city street—but those same sounds can plainly be heard over much larger distances. This effect is caused by "a unique combination of factors such as wind, weather, temperature, land topography, forest or other vegetation, and elevation," we read. For example, "battle sounds from Gettysburg fought on July 1, 2, and 3, 1863 could be heard over one hundred miles away in Pittsburgh, but were not heard only ten miles from the battlefield."

Without my own access to contemporary accounts of these battles and their acoustic shadows—sonic phantom limbs haunting distant landscapes—I simply have to trust the accounts that I'm quoting from here; nonetheless, these stories are fascinating. "More than 91,000 men were engaged in battle at Gaines's Mill, Virginia on June 27, 1862," for instance. "Confederate commanders and troops were less than two miles from the battlefield and could plainly see the smoke and flashes from the guns and artillery, but not a sound could be heard of the battle for two hours. Strangely, the battle sounds from the Battle of Gaines's Mill were easily heard in Staunton, Virginia over one hundred miles away."

The unexpected atmospheric reflection of sound, and sound's complicated relationship with certain topographies, levels of humidity, climatic systems, and more presents an amazing—if impossibly complex—dimension to the future of urban design and landscape architecture. Could 5th Avenue be retrofitted to cultivate acoustic shadows—or might a neighborhood in eastern Brooklyn someday find itself overhearing distant traffic events and individual human conversations that have been carried on the winds from Midtown, acoustic effects soon traced back to the mirage-like venting of a new steam plant on the East River?

This also makes me wonder if instances of ghostlike visitation in ancient times—a king crazed by invisible whispers in his fortified tower bedroom, a city cursed by nocturnal voices, a village terrified by bodyless beasts unseen by any hunter—might actually have been examples of acoustic shadows. How could acoustic shadows be archaeologically and historically investigated without exactly reproducing the landscape topography and climatic conditions of the time?

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Nice. Vaguely related, crowd-mapping the sound of #pdxboom to discover the location of a pipebomb in Portland: Boom! Tweets & Maps Swarm to Pinpoint a Mysterious Explosion. ("A topographic view of the map made some inclined to believe that cliffs across the river and low-hanging clouds combined to make the sound travel as far across the city and in the direction that it did.")

I grew up not too far from Staunton - near the Shenandoah Valley, VA. The example would make sense, as the soft limestone and clay foothills of the Appalachians and North-South lines of the parallel valley walls would channel sound unusually. The valley itself has a unique weather system as well.

It's a beautiful concept when extracted from the violence of war... how a soundscape could be designed on the scale of a regional landscape.

The lower the frequency (meaning more bass), then the longer these waves can travel and stay intact. With hills and valleys in the right place, you could pump some serious Einstürzende Neubauten across county lines.

Like Steve Goodman notes in his book, there is a politics to frequency which has largely been ignored. What I wonder is how might political lines be redrawn by 20 Hz ribbons of jet sound?

This is most probably due to an acoustic phenomenon knows as temperature inversion.

“Under conditions of a temperature inversion (temperature increasing with increasing height), the sound waves will be refracted downwards, and therefore may be heard over larger distances. This frequently occurs in winter and at sundown. For instance, the Nine O'Clock Gun in Vancouver has been heard up to 45 miles away under the proper atmospheric conditions.”[source]

It’s difficult to predict but as you can see from the diagram on that page can lead to things sounding louder further away.

Tangentially related: The Canadian sound artist Christof Migone created a piece called Quieting in 2000 based off field recordings of the daily cannon firing in Halifax, Nova Scotia. An excerpt from the statement for the piece:

"In my preparations for this project, I made several recordings from different positions in the city, but I only used one for the cd. I initially intended to do a piece combining the various recordings, but I was stuck on one in particular. To this day, I am startled every time I hear this particular firing.

In between the recording in 1996 and working on the cd in the Summer of 2000, I had periodically tried to use it, but I could never find the right form, everytime it was placed beside or alongside something, it would annihilate itself along with anything surrounding it. I finally realized that it had to stand on its own. And so in thinking of how one would create that possibility in the listening experience, I thought of putting just that brief recording in the middle of the cd, preceded and followed by nothing, just silence, so as to further amplify the sound of the shot."

One echo magnified - if not through an increase in volume than through recontextualization.

The Battle of Bunker Hill was heard more than 100 miles away in the Upper Connecticut River Valley (Lebanon and Hanover, N.H.) according to a 1775 letter and other sources. "Last Saturday and Sabbath we heard the noise of cannon, -- we suppose at Boston, -- and are now impatient to be informed of the occasion and event." That's not to say it wasn't also heard by those in the intervening locales.

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BLDGBLOG ("building blog") is written by Geoff Manaugh. The opinions expressed here are my own; they do not reflect the views of my friends, editors, employers, publishers, or colleagues, with whom this blog is not affiliated.